Measuring heart and pulse rates in living subjects has become a valuable tool during physical exercise and for health monitoring. The heart rate and pulse rate of a subject are related. Heart rate may be defined as the number of heart contractions over a specific time period, usually defined in beats per minute. A pulse is defined as the rhythmical dilation of a blood vessel produced by the increased volume of blood forced through the vessel by the contraction of the heart. Since heart contractions normally produce a volume of blood that can be measured as a pulse, heart rate and pulse rate are ideally the same. However, a pulse or pulse rate may differ from the heart rate during irregular heart beats or premature heart beats. In this case, a heart contraction may not force enough blood through a blood vessel to be measured as a pulse.
A pulse rate is measured by counting the rate of pulsation of a subject's artery. The heart rate is measured by sensing the electrical activity of the heart based on electrocardiograms (for example EKG or ECG). Heart and pulse rates may be monitored for a variety of reasons. Individuals who want to increase their endurance or performance may wish to exercise while maintaining target heart rates. Conversely, subjects with a history of heart disease or other heart related condition should avoid exceeding a certain heart or pulse rate to reduce unnecessary strain on their heart.
Pulse rate can be measured at the wrist. The shallow depth of the radial artery in the wrist offers a number of advantages for achieving continuous pulse detection at the wrist. However, some prior wrist-based monitors have disadvantages. For example, prior sensors that monitor pressure pulses in the wrist have not been effective. Pressure pulses are attenuated by the tissues between the artery and the sensor. Most of the high frequency signal components are lost because of the attenuation. Additionally, muscle movement may create substantial low frequency noise at the pressure sensors. The low frequency noise signals make reliable identification of low frequency blood pressure pulses very difficult.
Ultrasonic monitors using sonar technology were developed to overcome low frequency noise signal problems. Ultrasonic monitors transmit ultrasonic energy as a pulse signal. When a power source drives a transducer element, such as a piezoelectric crystal, to generate the pulse signal, the ultrasonic pulse signal is generated in all directions, including the direction of the object to be measured (such as a blood vessel). The portion of the ultrasonic pulse signal reaching the vessel is then reflected by the vessel. When the blood vessel experiences movement, such as an expansion due to blood flow from a heart contraction, the reflected pulse signal experiences a frequency shift, also known as the Doppler shift.
When either the source of a sonar or ultrasonic signal or the observer of the signal is in motion, an apparent shift in frequency results. The shift in frequency is known as the Doppler effect. If R is the distance from the ultrasonic monitor to the blood vessel, the total number of wavelengths λ contained in the two-way path between the ultrasonic monitor and the target is 2R/λ The distance R and the wavelength λ are assumed to be measured in the same units. Since one wavelength corresponds to an angular excursion of 2π radians, the total angular excursion Φ made by the ultrasound wave during its transit to and from the blood vessel is 4πR/λ radians. When the blood vessel experiences movement, R and the phase Φ are continually changing. A change in Φ with respect to time is equal to a frequency. This is the Doppler angular frequency Wd, given by
      W    d    =            2      ⁢      π      ⁢                          ⁢              f        d              =                            ⅆ          Φ                          ⅆ          t                    =                                                  4              ⁢              π                        λ                    ⁢                                    ⅆ              R                                      ⅆ              t                                      =                              4            ⁢            π            ⁢                                                  ⁢                          V              r                                λ                    where ƒd is the Doppler frequency shift and Vr is the relative (or radial) velocity of target with respect to the ultrasonic monitor.
The amount of the frequency shift is thus related to the speed of the moving object from which the signal reflects. Thus, for heart rate monitor applications, the flow rate or flow velocity of blood through a blood vessel is related to the amount of Doppler shift in the reflected signal.
A piezoelectric crystal may be used in a monitor both as the power generator and the signal detector. In this case, the ultrasonic energy is emitted in a pulsed mode. The reflected signal is then received by the same crystal after the output power source is turned off. The time required to receive the reflected signal depends upon the distance between the source and the object. Using a single crystal to measure heart rates requires high speed power switching due to the short distance between source and object. In addition, muscle movement generates noise that compromise the signal-to-noise-ratio in the system. The muscle movement noise has a frequency range similar to the frequency shift detected from blood vessel wall motion. Therefore, it is very difficult to determine heart rates with this method.
In some ultrasonic signal systems, two piezoelectric elements are used to continuously measure a pulse. The two elements can be positioned on a base plate at an angle to the direction of the blood flow. In continuous pulse rate measurement, the Doppler shift due to blood flow has a higher frequency than the shifts due to muscle artifacts or tissue movement. Therefore, even if the muscle motion induced signals have larger amplitudes, they can be removed by a high pass filter to retain the higher frequency blood flow signals. The disadvantages of continuous mode over pulsed mode are higher cost and more power consumption
In addition to ultrasound, other technologies have been used to monitor a subject's heart rate or pulse rate. These technologies include EKG, oximeters, radio frequency, and laser. Each of these technologies has its own disadvantages in measuring heart rates and pulse rates.
EKG signals are commonly used in medical environments to diagnose heart diseases and to calculate a patient's heart rate. To implement EKC technology, EKG electrodes are usually placed on patient's chest or limbs. Once placed, the electrodes communicate data to a processing device. The processing device may be a stand-alone machine, a wrist worn device, or some other device. The disadvantage with EKG technology is that it is used with a chest strap to monitor the subject's heart. It is not practical for use in a wrist worn device without a chest strap.
Oximeters which monitor oxygen content in a subject's blood can provide heart rate information as a byproduct. An oximeter directs infrared light or laser light at a subject's blood vessel. A monitor device then determines the amount of light absorption (or transmission of light energy) by the subject's blood. The change of light intensity with respect to time is used to compute the heart rate. The light emitter and detector is usually wrapped around a finger tip or clamped on an earlobe where arteries or arterioles can be found superficially.
Radio frequency (RF) technology uses the same Doppler principles as ultrasound-based heart rate monitors. Unlike ultrasound monitors, an RF signal transmitter and receiver do not need to have direct contact with the subject in order to efficiently send and receive signals to a subject. However, an RF-based monitor uses a Doppler signal with a much narrower band than ultrasound monitors. As a result, RF technology is not practical for wrist worn heart rate monitors used in the sports and fitness industry
Laser Doppler devices can be used to detect a heart rate based on the same Doppler principle used in ultrasound devices. However, the cost for using laser technology to monitor heart rates is very high. Also, the bandwidth used by laser devices is narrow, and therefore not practical for every day sports and fitness use.
For medical or industrial use where power consumption is not an issue, EKG, oximeter, radio frequency, and laser technologies can be applied to the subject to obtain continuous heart rate readings. However, for the portable, wearable and battery driven heart rate monitors which are popular in sports and fitness use, some of the above technologies are not practical.
For example, EKG based heart rate monitors are widely used for sports and fitness applications. This widespread use of EKG technology is because an EKG electrode is a passive device that requires no power. The only power consumption in an EKG based monitor is in the electronic circuit that processes the EKG signals received from the heart. Therefore, a standard lithium coin battery is suitable for use in these devices. This technology, however, requires the use of a chest strap to achieve continuous monitoring.
Other technologies, including the oximeter, radio frequency, and laser technologies mentioned above, require power to drive transmitting and receiving components. The power is required regardless of where the transducers are placed on the body (e.g., finger tip, earlobe, temple, neck, wrist, or other body location where blood pulse can be found fairly easily). For heart rate monitors using these technologies, it is desirable to reduce the power consumption of the device.